The present invention relates to the removal of a particular contaminant from a waste water stream containing substantially greater amounts of another contaminant. More particularly, the invention relates to the removal of arsenic or selenium from an aqueous solution containing a high background of sulfate, phosphate, chloride and/or fluoride. The invention further relates to a method for removing multiple contaminants from a waste water stream.
Contaminants such as selenium may be found in trace amounts of waste water from power plants, particularly those using lignite as a fuel source. While selenium is usually present in small amounts, averaging from about 0.1 to about 20 ppm, such levels are still too high to permit safe environmental discharge. Current drinking water regulations require selenium levels below about 0.01 mg/L or less than about 0.01 ppm. Selenium contaminants are also present in certain types of soils. With the exposure of these soils to water, even natural water depositories may be substantially contaminated with unsafe selenium levels. The Kesterson reservoir, near Fresno, Calif., for example, was recently tested for contamination. In addition to containing unacceptable amounts of selenium, water from this reservoir contained a substantially high background of sulfates as well. One particular water sample, for example, was analyzed to contain sulfate and selenium contaminants in a ratio of about 13,500 to 1.
For most aqueous environments, selenium exists in two different valence states: Se(IV) or Se.sup.4+, usually present as HSeO.sub.3.sup.-, or SeO.sub.3.sup.2- (selenite); and Se(VI) or Se.sup.6+, usually present as SeO.sub.4.sup.2- (selenate). Both valences of selenium are not typically responsive to the same removal means. For example, it is well known to remove Se(IV) from a waste water using ferric chloride or alum in a coagulation technique. The same treatment does not significantly affect Se(VI) concentrations, however. In an EPA report entitled "Selenium Removal from Ground Water Using Activated Alumina", Trussell et al. stated that for the same operating conditions and water compositions, Se(VI) was removed at about one-tenth the rate as Se(IV). This difference in adsorption behavior may be due, in part, to the chemical similarity of SeO.sub.4.sup.2- and sulfate ions. As such, there is no predictable preference for removing Se(VI) or selenate from a high background of sulfates, phosphates, chlorides and/or fluorides.
Contact with an activated alumina is known to remove selenites from a solution. This method even serves as an effective pretreatment according to the present invention. In a paper entitled "Removal of Inorganic Selenium from Drinking Water by Activated Alumina", Kreft et al. observed that this adsorbent's capacity for removing Se(IV) was much higher than its capacity for Se(VI) removal. Kreft et al. further noted that sulfates and biocarbonates heavily interfere with Se(VI) adsorption at concentrations greater than 100 mg/L. An extensive thermochemical study by Novak et al. (Lewis Publ., Inc., Chelsen, Mich., 1987: "Mechanisms of Metal Ion Adsorption on Activated Alumina") confirmed the observations of Kreft and provided theoretical explanations based on heats of adsorption.
Arsenic occurs in one of two valence states, either As.sup.3+ or As.sup.5+. Like selenium, arsenic removal from waste water becomes complicated when high backgrounds of other contaminants are also present. For example, in the aqueous streams associated with phosphate mining or with the production of certain phosphate fertilizers and pesticides, environmentally unacceptable amounts of As(III) and As(V) exist in a substantially high phosphate or sulfate background. Depending on the selectivity of the adsorbent used, a substantial amount of the background contaminant may have to be adsorbed before arsenic is sufficiently removed from the stream. It would therefore be desirable to develop a process which may selectively remove selenium or arsenic anions from waste water when substantially greater quantities of other contaminants, such as phosphates, sulfates, fluorides and chlorides, are also present.
Various methods are known for removing selenium from particular aqueous solutions. For example, Clark et al. U.S. Pat. No. 3,914,375 shows a method for removing selenium from a copper solution. Marchant U.S. Pat. No. 3,933,635 removes selenium from acidic waste water and Kakuta et al. U.S. Pat. No. 3,966,889 claims a process for recovering selenium from combustion waste gases. Shawl et al. U.S. Pat. No. 4,130,633 shows a method for removing and recovering selenium from a urethane solution. In Weir et al. U.S. Pat. Nos. 4,222,999, 4,330,508 and 4,374,808, various processes for removing Se(IV) and/or Se(VI) from copper sulfate or copper-nickel sulfate solutions are disclosed.
Hofirek U.S. Pat. No. 4,377,556 claims another process for removing dissolved selenium from a copper sulfate solution. The process includes treating the solution to a temperature of at least about 140.degree. C. with a stoichiometric excess of sulfur dioxide or sulfite solution. In Chou et al. U.S. Pat. No. 4,544,541, a process is shown for removing Se(VI) from a nickel, cobalt or copper sulfate solution, said process including treating the solution with an effective amount of a metal hydride, metal borohydride or ammonia borane, and with finely divided nickel, cobalt or copper metal. Baldwin et al. U.S. Pat. No. 4,405,464 shows another process for reducing the selenium concentration of an aqueous solution by admixing a quantity of metallic iron into the solution for reducing at least some Se(VI) ions to the (+4) oxidation state.
Two commonly assigned references, U.S. Pat. Nos. 4,519,912 and 4,519,913, disclose removing water soluble selenium by passing solution through a treatment zone containing a population of at least one bacteria from the genus Clostridium. Downing et al. U.S. Pat. No. 4,725,357 removes dissolved selenium from waste water by treating the water in a reactor containing a microbial biomass and a biomass nutrient. This treatment, which occurs in the substantial absence of free oxygen, captures selenium in particles larger than those originally present.
Other general methods are known for separating individual anions from solution with hydrotalcite. Manabe et al. U.S. Pat. No. 4,458,030, for example, shows a composite adsorbent useful for removing AsO.sub.4.sup.-3 from an aqueous solution. The adsorbent consists of 5-95 wt. % hydrotalcite or calcined hydrotalcite and a balance of activated carbon. Sood U.S. Pat. No. 4,752,397 claims a process for purifying an aqueous solution containing one or more heavy metal ions, including arsenic and selenium, by passing the solution through an adsorbent containing at least about 20 wt. % activated hydrotalcite.
In Japanese Patent Application No. 54-24993, there is shown a method for removing certain individual ions, such as arsenic and selenium (among others), from a solution. The method uses hydrotalcite having the formula XMgO.Al.sub.2 O.sub.3.YCO.sub.2.ZH.sub.2 O wherein X represents 1-10, Y represents 0.4-2.5 and Z represents 4-20. The method may also proceed with calcined hydrotalcite or with a mixture of both products. In a 1986 article by Sato et al. entitled, "Adsorption of Various Anions by Magnesium Aluminum Oxide (Mg.sub.0.7 Al.sub.0.3 O.sub.1.15)", the adsorptive behavior of a thermally dehydrated synthetic hydrotalcite was studied relative to certain divalent anions including phosphates and sulfates. Based on these references, the Kreft et al. teaching that Se(VI) adsorption onto activated alumina is hampered by the presence of excess sulfate, and on individual adsorption isotherms for hydrotalcite-based products, it is clear that such products may be used to individually remove arsenic, selenium, phosphate or sulfate from a solution. None of the foregoing references teach or show using activated hydrotalcite to remove multiple contaminants from a solution, especially one which contains a substantial background of sulfate or phosphate, and only minor amounts of arsenic or selenium. Rather, from the individual isotherms for activated hydrotalcite accompanying this invention (FIGS. 1A through 1C), one might conclude that lower contamination levels of arsenic and selenium would be removed at a much slower kinetic rate than the high background of other contaminant. One might further conclude that substantially all divalent anions present would have to be removed before this adsorbent would remove any As(III)/As(V) or Se(IV)/Se(VI) present. As learned from this invention, however quite the opposite is true.